I had a stupid line of code:
Guid guid = value as Guid;
Resharper was fine with this, but the compiler complains with:
ERROR #: CS0077
The as operator must be used with a reference type ('System.Guid' is a value
The "as Guid" was redundant in this case (it was a property set method and
the property was a Guid type), but still, Resharper should have caught this.